As a result of their chemical and structural homogeneity providing a lack of local electrochemically active sites, metallic glasses are characterized by improved corrosion resistance when compared to typical crystalline metals. For example, in a study conducted by Morrison et al., the corrosion resistance of a Zr—Ti-based bulk metallic glass (BMG) in a phosphate-buffered saline solution was compared against that of common crystalline biomaterials and was found to be better than 316L stainless steel and comparable to Ti-6Al-4V and CoCrMo. (See, Morrison, M. L., et al., Intermetallics, 12, 1177 (2004), the disclosure of which is incorporated herein by reference.) Most metallic glasses are, therefore, expected to behave well in corrosive environments, and little research has been done to identify whether these corrosive properties can be improved. As a result, BMGs having corrosion resistant properties that meet or exceed the most corrosion resistant conventional materials, such as stainless steels, Ti-alloys, and CoCr-based alloys have not been reported.
In addition, owing to the fact that the compositions of known BMGs have not been optimized for corrosion resistance, many BMG compositions are found to behave poorly in some corrosive environments. BMG compositions generally require the presence of Late Transitions Metals (LTM), either as base metals or as alloying additions. Ni and Cu are in fact the most commonly found LTM's in BMG's, as most of the known BMG compositions contain either Ni or Cu or both. For example, presently one of the most widely available commercial BMGs is a Ni and Cu containing Zr—Ti-based materials sold under the tradename VITRELOY by LiquidMetal technologies, Inc. (See, e.g., U.S. Pat. No. 5,288,344; and Peker, A. & Johnson, W. L., Applied Physics Letters, 63, 2342 (1993), the disclosures of which are incorporated herein by reference.) Aside from being excellent additions to BMG compositions, Ni and Cu are generally acceptable elements in many conventional engineering applications. One of the exceptions could be corrosion. Recognizing that Ni and Cu are highly electronegative, one would expect alloys containing Ni and Cu to perform rather poorly under corrosive environments, particularly in compositions where they are combined with highly electropositive metals such as Zr, Ti, or Be. By these considerations, it is therefore conceivable that combining Zr, Ti and Be with Ni and Cu, as in VITRELOY would result in a BMG alloy whose resistance against certain corrosive reactions would not be as high as one might expect.
Aside from corrosive effects of Ni and Cu alloy additions, the relatively high electronegativity of these elements gives rise to other undesirable effects which could be of great concern in certain applications, such as for instance in biological applications. Specifically, owing to their high etectronegativity, Ni and Cu have the possibility of existing as free radicals in the blood stream. In turn, these free radicals are notorious triggers for severe adverse biological reactions. In consequence, Ni and Cu are widely regarded as non-biocompatible, as they have been associated with severe adverse biological reactions. (See, Geurtsen, W., Critical Reviews in Oral Biology & Medicine, 13, 35 (2005), the disclosure of which is incorporated herein by reference.) As a result, the vast majority of the known Zr—Ti-based BMGs compositions cannot qualify as biocompatible and hence their use in biological applications may be limited.
Accordingly, a need exists for a class of Zr—Ti-based BMGs that have improved corrosion resistance properties, and preferably that are Ni and Cu free to ensure good biocompatibility.